The present invention relates to PC cards used to provide additional circuitry and external connections for computers and other electrical devices. PC cards (cards with printed circuits) are commonly made in accordance with PCMCIA (Personal Computer Memory Card International Association) specifications which define, among other things, the dimensions of such card. One of the critical dimensions is the maximum thickness of the cards, the maximum thickness being 5 mm for the common Type II PC cards, which permits the cards to be inserted into a standard receiving slot of an electrical device such as a laptop computer. Such PC cards normally include a housing having top and bottom sheet metal cover parts (which may be integrally joined at one end), a printed circuit board holding circuit components and contained within the housing, and a front connector at a front end of the card. The front connector mates with a device connector at the front end of the insertion slot to connect to circuitry on the circuit board. The housing is intended to provide shielding, or screening, against electromagnetic radiation passing into or out of the PC card.
Circuitry, including circuit components (e.g. integrated circuits, capacitors, conductors, and resistors, which all affect currents passing therethrough) sometimes require electromagnetic isolation from other circuitry to prevent cross coupling. Such shielding or screening must remain intact after mechanical flexing of the PC card, which normally occurs in use. PCMCIA specifications describe a "torque test" and "bend test" which cards must survive to simulate this phenomenon. One way of providing screening is to employ a number of shielding inner covers that fit over interfering circuitry and which are coupled to a grounding trace on the circuit board. Such covers are unreliable due to their resistance to flexing and a consequent risk of damage to the circuit board during flexing of the PC card.
Applicants have tried to provide internal shielding by using conductive elastomer to form a wall around the circuitry to be screened. Such elastomer wall is sandwiched between a grounding trace on the circuit board and a cover part, and is compressed between them. In this way, the elastomer wall and the cover part form an effective screen for the circuitry lying within the wall. A problem with this construction is that it does not provide sufficient compression force between the cover part and circuit board to ensure reliable low resistance contact, without bulging out the cover part. The cover parts are typically formed of thin sheet metal such as stainless steel, to limit the overall thickness of the card while leaving space for the circuit board and the circuit components. As a result, even a low force can result in the cover outer surfaces becoming convex so the cover bulges. Bulging covers can result in the card thickness increasing beyond the specified maximum thickness, as well as resulting in a card appearance which is unsightly and which suggests damage. The present invention seeks to provide a solution to the above problem.